Dynamic transmission protection in the presence of multiple modulation schemes

ABSTRACT

A technique is disclosed for re-enabling transmission protection at enhanced stations while in the presence of high, enhanced modulation traffic conditions once protection has been disabled. Transmission protection allows enhanced modulation stations to co-exist with legacy modulation stations on a shared-communications channel. The problem being solved is when transmission protection is set to inactive and the Orthogonal Frequency Division Multiplexing (i.e., enhanced modulation) traffic load is high, legacy traffic is likely to collide repeatedly with Orthogonal Frequency Division Multiplexing transmissions, with the result that the access point does not notice that a legacy station has become active again. Consequently, the access point does not activate transmission protection. The present invention addresses the problem by defining access point mechanisms that are capable of re-enabling transmission protection, once protection has been disabled, in the presence of high, enhanced modulation traffic conditions.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a Continuation of U.S. application Ser. No.10/688,527, filed Oct. 17, 2003, which claims priority from U.S.Provisional Application 60/439,697, filed Jan. 13, 2003, both of whichare incorporated herein by reference in their entirety.

FIELD

The present disclosure relates to telecommunications in general, and,more particularly, to wireless local area networks.

BACKGROUND

FIG. 1 depicts a schematic diagram of a portion of wireless local areanetwork 100 in the prior art. Local area network 100 comprises stations101, 102-1, and 102-2. Stations 101, 102-1, and 102-2 use ashared-communications channel to communicate among themselves and onlyone of the terminals can transmit into the channel at a time. When twoterminals transmit into the channel at the same time, the result is acacophony and both transmissions are garbled.

Therefore, a technique called Carrier Sense Multiple Access is used bythe stations to coordinate when each of them transmits. In accordancewith this technique, each radio listens to the shared-communicationschannel and waits to transmit until the channel is quiet (i.e., no otherstations are transmitting). Carrier Sense Multiple Access is similar tothe way in which each person in a group of polite people waits to speakuntil the person speaking is finished.

Station 101 can transmit and receive using:

i. a first modulation scheme.

In contrast, stations 202-1 and 202-2 can transmit and receive using:

i. the first modulation scheme, and

ii. a second modulation scheme.

Because stations 102-1 and 102-2 can use a superset of the modulationsschemes available to station 101, they are called “enhanced” stations.In contrast, station 101 is called a “legacy” station.

The modulation schemes available to a legacy station are called “legacymodulation schemes” and the modulation schemes available to an enhancedstation are called “enhanced modulation schemes.”

Any two stations that need to communicate must do so in accordance witha modulation scheme that is available to both of them. Therefore, twoenhanced stations can communicate with any of the enhanced modulationschemes, but any communication involving a legacy station must use alegacy modulation scheme.

Stations 102-1 and 102-2 communicate with each other using the secondmodulation scheme when possible because it enhances communicationthroughput in comparison to the first modulation scheme. One effect ofusing the second scheme, however, is that station 101 cannot detect whenstations 102-1 and 102-2 are communicating (i.e., stations 102-1 and102-2 are essentially invisible to station 101 when stations 102-1 and102-2 are communicating using the second modulation scheme). This cancause station 101 to transmit when stations 102-1 and 102-2 arecommunicating, which causes all of the transmissions to be garbled.

To address this problem, a method called “transmission protection” isknown in the prior art to prevent legacy stations from transmittingwhile transmissions using the second modulation scheme are in progress.

In accordance with transmission protection, an enhanced station that isabout to transmit a frame using the second modulation scheme firsttransmits a short frame using the first modulation scheme. This shortframe is detectable by the legacy stations in the area.

A duration field in the short frame contains a value that indicates howlong the legacy terminals should refrain from transmitting, and thefield is populated with a duration that is long enough to cover thelength of time for transmissions of frames using the second modulationscheme. The duration information inside the Request-to-Send orClear-to-Send frame activates a virtual carrier sense mechanism in thelegacy stations, which will not transmit, as a result, during theprotected, subsequent second transmission.

A mechanism is also known in the prior art for notifying all of theenhanced stations in the network when to use and when not to usetransmission protection. In accordance with this mechanism, one enhancedstation has the capability to activate and deactivate transmissionprotection in the other enhanced stations by signaling whether or notprotection must be used.

If transmission protection is disabled and there is a heavy traffic loadusing the second modulation scheme, transmissions from legacy stationswill repeatedly collide with transmissions using the second modulationscheme and there might be an unreasonable delay before an enhancedstation notices that a legacy station is trying to transmit. As aresult, there might be a delay in activating transmission protection inthe enhanced terminals.

Therefore, the need exists for a technique for reasonably activatingtransmission protection in enhanced terminals.

SUMMARY OF THE INVENTION

The present invention addresses this problem by defining access pointmechanisms that are capable of re-enabling transmission protection, onceprotection has been disabled, in the presence of heavy, enhancedmodulation (e.g., Orthogonal Frequency Division Multiplexing, etc.)traffic.

In accordance with the first illustrative embodiment of the presentinvention, when a legacy communication station is in a power save state,the access point periodically transmits a Null frame to that station.When the station “awakens” (i.e., has exited the power save state) andresponds with an Acknowledgement frame, the access point re-enablestransmission protection.

In accordance with the second illustrative embodiment of the presentinvention, when a legacy station becomes active and sends a message tothe access point, the access point immediately activates transmissionprotection by sending a Probe-Response frame, comprising a protectionstatus field that is set to active, to the enhanced stations. This is adifferent use of the Probe-Response frame than in the prior art becausethe frame is broadcast proactively, as opposed to being unicast inresponse to a previously-sent Probe-Request frame.

In accordance with the third illustrative embodiment of the presentinvention, the access point intermittently switches between enablingtransmission protection and disabling transmission protection atenhanced stations. The intervals during which protection is activatedfacilitate the transmission of any pending legacy modulation frame.

In accordance with the fourth illustrative embodiment of the presentinvention, the access point repeatedly transmits an enhanced modulationframe that inhibits transmissions by enhanced stations for a portion ofthe time period between repeated enhanced modulation frametransmissions. This gives legacy stations the opportunity to transmit asneeded because they do not detect the enhanced inhibiting frame.

The illustrative embodiment comprises: determining a power save statusof a first station wherein said first station communicates via ashared-communications channel in accordance with a first modulationscheme; and enabling transmission protection at a second station viasaid shared-communications channel wherein said enabling is dependent onsaid power save status.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic diagram of a portion of wireless local areanetwork 100 in the prior art.

FIG. 2 depicts a schematic diagram of a portion of local area network200 in accordance with the illustrative embodiment of the presentinvention.

FIG. 3 depicts a block diagram of the salient components of enhancedstation 202-2 in accordance with the illustrative embodiment of thepresent invention.

FIG. 4 depicts a message flow diagram in accordance with the firstillustrative embodiment of the present invention.

FIG. 5 depicts a message flow diagram in accordance with the secondillustrative embodiment of the present invention.

FIG. 6 depicts a message flow diagram in accordance with the thirdillustrative embodiment of the present invention.

FIG. 7 depicts a message flow diagram in accordance with the thirdillustrative embodiment of the present invention wherein enhancedstation 202-2 receives a legacy modulation frame from legacy station 201during a protected period.

FIG. 8 depicts a message flow diagram in accordance with the thirdillustrative embodiment of the present invention wherein enhancedstation 202-2 receives a legacy modulation frame from legacy station 201during an unprotected period.

FIG. 9 depicts a message flow diagram in accordance with the fourthillustrative embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 2 depicts a schematic diagram of wireless local area network 200 inaccordance with the illustrative embodiment of the present invention.Local area network 200 operates in accordance with the IEEE 802.11 setof protocols and comprises communication stations 201, 202-1, and 202-2,and hosts 203-1 and 203-2, interconnected as shown.

Station 201 transmits and receives using:

i. Complementary Code Keying, and

ii. Barker modulation.

In contrast, stations 202-1 and 202-2 transmit and receive using:

i. Complementary Code Keying,

ii. Barker modulation, and

iii. Orthogonal Frequency Division Multiplexing.

Because stations 202-1 and 202-2 can use a superset of the modulationsschemes available to station 201, they are called “enhanced” stationswhereas station 201 is called a “legacy” station.

The modulation schemes available to a legacy station are called “legacymodulation schemes” and the modulation schemes available to an enhancedstation are called “enhanced modulation schemes.” Any two stations thatneed to communicate must do so in accordance with a modulation schemethat is available to both of them. Therefore, two enhanced stations cancommunicate with any of the enhanced modulation schemes, but anycommunication involving a legacy station must use a legacy modulationscheme.

For the purposes of this specification, a frame that is transmittedusing a legacy modulation scheme is referred to as “legacy modulationframe,” and a frame that is transmitted using an enhanced modulationscheme is referred to as “enhanced modulation frame.”

It will be clear to those skilled in the art, after reading thisspecification, how to make and use embodiments of the present inventionthat operate in accordance with other protocols and modulation schemes.Furthermore, it will be clear to those skilled in the art, after readingthis specification, how to make and use embodiments of the presentinvention that use a wireline or tangible shared-communications channel.And still furthermore, it will be clear to those skilled in the art,after reading this specification, how to make and use embodiments of thepresent invention with any number of legacy stations and any number ofenhanced stations.

Enhanced station 202-2 is an access point and, as such, enables stations201 and 201-1 within local area network 200 to communicate with devicesin other communications networks. Because station 202-2 is an accesspoint, stations 201 and 202-1 communicate with each other throughenhanced station 202-2, because enhanced station 202-2 coordinates thecommunications on local area network 200. The salient details ofenhanced station 202-2 are described below and with respect to FIG. 3.

Legacy station 201 comprises the radio that enables host 203-1 tocommunicate via the shared-communications channel. Legacy station 201 iscapable of receiving a data block from host 203-1 and transmitting overthe shared-communications channel one or more data frame comprising thepayload portion of the data block. Legacy station 201 is also capable ofreceiving one or more data frames from the shared communications channeland sending to host 203-1 a data block comprising payload portion of thedata frames. It will be clear to those skilled in the art how to makeand use legacy station 201 and host 203-1.

Enhanced station 202-1 and host 203-2 have a relationship similar tothat described for legacy station 201 and host 203-1. It will be clearto those skilled in the art how to make and use stations 202-1, andhosts 203-1 and 203-2.

FIG. 3 depicts a block diagram of the salient components of enhancedstation 202-2, the access point, in accordance with the illustrativeembodiment of the present invention. Enhanced station 202-2 comprises:receiver 301, processor 302, memory 303, and transmitter 304,interconnected as shown.

Receiver 301 is a circuit that is capable of receiving frames fromanother station via the shared-communications channel, in well-knownfashion, and of forwarding them to processor 302. It will be clear tothose skilled in the art, after reading this specification, how to makeand use receiver 301.

Processor 302 is a general-purpose processor that is capable ofperforming the tasks described below and with respect to FIGS. 4 through9. It will be clear to those skilled in the art, after reading thisspecification, how to make and use processor 302.

Memory 303 is capable of storing programs and data used by processor302. It will be clear to those skilled in the art how to make and usememory 303.

Transmitter 304 is a circuit that is capable of receiving frames fromprocessor 302, in well-known fashion, and of transmitting them toanother station via the shared-communications channel in accordance withan enhanced modulation scheme. It will be clear to those skilled in theart, after reading this specification, how to make and use transmitter304.

FIG. 4 depicts a message flow diagram in accordance with the firstillustrative embodiment of the present invention. When the message flowdiagram in FIG. 4 begins, transmission protection is disabled for theenhanced station in local area network 200.

Enhanced station 202-2 (i.e., the access point) tracks the power savestatus of the legacy stations (e.g., legacy station 201, etc.) presentin local area network 200. The power save state is defined as a state inwhich the transmitter (or both the transmitter and receiver) of astation is powered down, and the power save status is an indication ofwhether or not a legacy station is in the power save state.

With message 401, enhanced station 202-2 transmits to legacy station 201a Null frame to determine the power save status of legacy station 201. ANull frame, as defined in the IEEE 802.11 protocols, is a frame with anempty payload, which is answered with an Acknowledgement frame by thereceiving station and then discarded. In some alternative embodiments ofthe present invention, message 401 can be a Request-to-Send frame or adata frame instead. When legacy station 201 is in the power save state,then enhanced station 202-2 does not receive a response, as is the casefor message 401. Enhanced station 202-2 then attempts to determine thepower save status of other legacy stations in the network that itcoordinates.

With message 402, enhanced station 202-2 transmits to legacy station 201another Null frame as part of continually tracking the power save statusof legacy stations. When legacy station 201 is not in the power savestate, legacy terminal 201 responds with message 403, which is anacknowledgement that legacy terminal 201 has received message 402.Message 403 is an Acknowledgement frame or a Clear-to-Send framedepending, as will be clear to those skilled in the art, on whethermessage 402 is a Null frame, a Request-to-Send frame, or a data frame.

When station 202-2 receives message 403 enhanced station 202-2immediately activates transmission protection for itself and broadcastsmessage 404 to activate transmission protection in the other enhancedstations in the network. Message 404 is a management frame, which can befor example, a Beacon frame or a Probe-Response frame with protectionstatus set to active. The protection status can be represented by theUse_Protection bit for IEEE 802.11 protocols, for example. It will beclear to those skilled in the art how to broadcast a management frame tostations in local area network 200.

In some alternative embodiments of the present invention, station 202-2broadcasts the management frame using a legacy modulation scheme. In yetsome other alternative embodiments of the present invention, station202-2 first transmits a control frame (e.g., Request_to_Send,Clear_to_Send, etc.) using a legacy modulation scheme, specifying aduration that covers a management frame that station 202-2 subsequentlybroadcasts using either an enhanced or legacy modulation scheme.

FIG. 5 depicts a message flow diagram in accordance with the secondillustrative embodiment of the present invention. When the message flowdiagram in FIG. 5 begins, transmission protection is disabled for theenhanced station in local area network 200.

FIG. 5 represents a scenario in which enhanced station 202-2 (i.e., theaccess point) detects a legacy station transmission, but cannot waituntil the next beacon frame transmission to update the enhancedstations' transmission protection.

With message 501, legacy station 201 transmits a frame to enhancedstation 202-2 when transmission protection is inactive, which message isindicative that a legacy station has become active.

As a result of the receipt of message 501, enhanced station 202-2immediately activates transmission protection for its own transmissions.Enhanced station 202-2 then with message 502 enables transmissionprotection for all enhanced stations in the network.

With message 502 enhanced station 202-2 broadcasts a Probe-Responseframe to all stations, including enhanced station 202-1 with theprotection status set to active. The protection status can berepresented by the Use_Protection bit for IEEE 802.11 protocols, forexample. The second illustrative embodiment of the present inventionuses the Probe-Response frame proactively and with a broadcast address,as opposed to using the Probe-Response as a unicast response, as isknown in the art, to an IEEE 802.11 Probe Request message.

In some alternative embodiments of the present invention, station 202-2broadcasts the Probe-Response frame using a legacy modulation scheme. Inyet some other alternative embodiments of the present invention, station202-2 first transmits a control frame (e.g., Request_to_Send,Clear_to_Send, etc.) using a legacy modulation scheme, that specifies aduration that covers a Probe-Response frame that station 202-2subsequently broadcasts using either an enhanced or legacy modulationscheme.

FIG. 6 depicts a message flow diagram in accordance with the thirdillustrative embodiment of the present invention. When the message flowdiagram in FIG. 6 begins, transmission protection is disabled for theenhanced station in local area network 200.

In accordance with the third illustrative embodiment, enhanced station202-2 (i.e., the access point) alternately switches between enablingtransmission protection and disabling transmission protection for bothits own transmissions and for all of the enhanced stations in local areanetwork. The intervals during which protection is activated facilitatethe transmission of any pending frames from legacy station 201.

With message 601, enhanced station 202-2 broadcasts a management frameindicating active protection status. The management frame can be aBeacon frame or a Probe-Response frame with the protection status fieldset to active, which directs the enhanced stations in local area network200 to enable transmission protection during a first time period.

In some alternative embodiments of the present invention, station 202-2broadcasts the management frame using a legacy modulation scheme. In yetsome other embodiments of the present invention, station 202-2 firsttransmits a control frame (e.g., Request_to_Send, Clear_to_Send, etc.)using a legacy modulation scheme that specifies a duration that covers amanagement frame that station 202-2 subsequently broadcasts using eitheran enhanced or legacy modulation scheme.

With message 602, enhanced station 202-2 broadcasts a management frameindicating inactive protection status. The management frame can be aBeacon frame or a Probe-Response frame with the protection status fieldset to inactive, which directs the enhanced stations in local areanetwork 200 to disable transmission protection during a second timeperiod.

Messages 603 and 604 are the same as messages 601 and 602, respectively.Enhanced station 202-2 alternately enables and disables transmissionprotection for enhanced station 202-1 and other enhanced stations withrespect to time. In some embodiments, this enabling and disabling isperiodic with respect to time, so that the length of the third timeperiod depicted in FIG. 6 is the same length as the first time period,and the length of the fourth time period is the same length as thesecond time period, and so on. In some other embodiments of the presentinvention, alternately enabling and disabling transmission protectioncan be sporadic with respect to time.

In still other embodiments, the enabling and disabling of transmissionprotection can be performed with respect to management framestransmitted. For example, enhanced station 202-2 can alternately enabletransmission protection in two successive management frames and disabletransmission protection in a third management frame. It will be clear tothose skilled in the art, after reading this specification, that anynumber of frames in which transmission protection is enabled can befollowed by any number of frames in which transmission protection isdisabled. This pattern can be periodic or sporadic with respect toframes transmitted by enhanced station 202-2.

FIG. 7 depicts a message flow diagram in accordance with the thirdillustrative embodiment of the present invention wherein enhancedstation 202-2 receives a legacy modulation frame from legacy station 201during a protected period. When the message flow diagram in FIG. 7begins, transmission protection is disabled for the enhanced station inlocal area network 200.

In accordance with the third illustrative embodiment, enhanced station202-2 (i.e., the access point) alternately switches between enablingtransmission protection and disabling transmission protection for bothits own transmissions and for enhanced station 202-1's 4829-2114-9968.1transmissions. The intervals during which protection is activatedfacilitate the transmission of any pending frames from legacy station201.

With message 701, enhanced station 202-2 broadcasts a management frameindicating active protection status. The management frame can be aBeacon frame or a Probe-Response frame with the protection status fieldset to active, thereby directing enhanced station 202-1, as well asother enhanced stations, to enable transmission protection for its owntransmissions during the first time period of length T₁.

With message 702, enhanced station 202-2 broadcasts a management frameindicating inactive protection status. The management frame can be aBeacon frame or a Probe-Response frame with the protection status fieldset to inactive, thereby directing enhanced station 202-1, as well asother enhanced stations, to disable transmission protection for its owntransmissions during the second time period.

With message 703, enhanced station 202-2 broadcasts a management frameindicating active protection status for the third time period.

With message 704, enhanced station 202-2 receives a legacy modulationframe from legacy station 201. In accordance with the illustrativeembodiment of the present invention, enhanced station 202-2 extends theprotected period to length T₂ as a result of having detected the legacymodulation frame, message 704. Length T₂, for example, can be greaterthan length T₁, the length of the protected period that follows message701.

With message 705, enhanced station 202-2 resumes alternately enablingand disabling protection by broadcasting a management frame indicatinginactive protection status for the fourth time period.

FIG. 8 depicts a message flow diagram in accordance with the thirdillustrative embodiment of the present invention wherein enhancedstation 202-2 receives a legacy modulation frame from legacy station 201during an unprotected period. Transmission protection is initiallyenabled for enhanced station 202-1 and other enhanced stations.

In accordance with the third illustrative embodiment, enhanced station202-2 (i.e., the access point) alternately switches between disablingtransmission protection and enabling transmission protection for bothits own transmissions and for enhanced station 202-1's transmissions.The intervals during which protection is activated facilitate thetransmission of any pending frames from legacy station 201.

With message 801, enhanced station 202-2 broadcasts a management frameindicating inactive protection status. The management frame can be aBeacon frame or a Probe-Response frame with the protection status fieldset to inactive, thereby directing enhanced station 202-1, as well asother enhanced stations, to disable transmission protection for its owntransmissions during the first time period of length T₁.

With message 802, enhanced station 202-2 broadcasts a management frameindicating active protection status. The management frame can be aBeacon frame or a Probe-Response frame with the protection status fieldset to active, thereby directing enhanced station 202-1, as well asother enhanced stations, to enable transmission protection for its owntransmissions during the second time period of length T₂.

With message 803, enhanced station 202-2 broadcasts a management frameindicating inactive protection status for the third time period.

With message 804, enhanced station 202-2 receives a legacy modulationframe from legacy station 201. In accordance with the illustrativeembodiment of the present invention, enhanced station 202-2 reduces thelength T₃ of the unprotected third time period as a result of havingdetected the legacy modulation frame of message 804. Length T₃, forexample, can be made shorter than length T₁, the length of the protectedperiod that follows message 801.

With message 805, enhanced station 202-2 effectively ends theunprotected period by broadcasting a management frame indicating activeprotection status. The length T₄ of the protected fourth period is madegreater than the length of T₂, the length of the previous protectedperiod (i.e., the second time period).

With message 806, enhanced station 202-2 resumes alternately enablingand disabling protection for set lengths (e.g., T₅ is made equal toT_(i), etc.) by broadcasting a management frame indicating inactiveprotection status for the fifth time period.

FIG. 9 depicts a message flow diagram in accordance with the fourthillustrative embodiment of the present invention. Transmissionprotection is initially disabled for enhanced station 202-1 and otherenhanced stations.

With message 901, enhanced station 202-2 (i.e., the access point)transmits a frame using the Orthogonal Frequency Division Multiplexing(i.e., enhanced) modulation scheme to enhanced station 202-1. ThisOrthogonal Frequency Division Multiplexing frame comprises a durationfield with a value that is used by all stations that receive the frameto set their network allocation vectors (NAV) and refrain fromtransmitting, in well-known fashion. During the time intervalcorresponding to the duration field value (depicted as being K timeunits in length), enhanced stations, including enhanced station 202-1,refrain from transmitting any frames and are muted.

Meanwhile, legacy station 201 is unable to detect this frame, becauselegacy station 201, as well as other legacy stations, is unable todetect Orthogonal Frequency Division Multiplexing modulations, and as aresult will not set its network allocation vector. Legacy station 201,therefore, has an opportunity to transmit while the enhanced stationsare muted.

With message 902, legacy station 201 transmits a frame to enhancedstation 202-2 using a legacy modulation scheme (e.g., Barker,Complementary Code Keying, etc.).

At time 903, the time interval ends for enhanced stations to be muted.

With message 904, enhanced station 202-1 is allowed to transmit a frameto enhanced station 202-2.

With message 905, the muting cycle begins again, starting with enhancedstation 202-2 transmitting a frame using the Orthogonal FrequencyDivision Multiplexing modulation scheme to enhanced station 202-1. ThisOrthogonal Frequency Division Multiplexing frame comprises a durationfield with a value that is used by all stations that receive the frameto set their network allocation vectors (NAV) and refrain fromtransmitting, in well-known fashion. During the time intervalcorresponding to the duration field value (depicted as being K timeunits in length), enhanced station 202-1, noting the duration fieldvalue, refrains from transmitting any frames.

At time 906, the time interval ends for enhanced stations to be muted.

The illustrative embodiment of the present invention takes advantage ofthe fact that when enhanced station 202-2 transmits a frame comprising aduration value using an enhanced modulation scheme, the legacy stations(e.g., legacy station 201, etc.) are unable to detect the frame and, asa result, continue to access the shared-communications channel asneeded. This is in contrast to earlier techniques disclosed outside ofthis specification in which a station transmits a frame comprising aduration value to mute both enhanced and legacy stations.

It is to be understood that the above-described embodiments are merelyillustrative of the present invention and that many variations of theabove-described embodiments can be devised by those skilled in the artwithout departing from the scope of the invention. It is thereforeintended that such variations be included within the scope of thefollowing claims and their equivalents.

1. A method comprising: determining, at an access point, a power savestatus of a first device configured to communicate in accordance with afirst modulation scheme, wherein determining the power save statuscomprises: transmitting a Null frame from the access point to the firstdevice and awaiting a response to the Null frame from the first device;and responsive to a determination that the first device is not in apower save state, (i) enabling transmission protection at the accesspoint; and (ii) transmitting, from the access point, a messagerequesting that a second device enable transmission protection, whereinthe second device and the access point are configured to communicate inaccordance with the first modulation scheme and a second modulationscheme.
 2. The method of claim 1, wherein determining the power savestatus of the first device further comprises: receiving one of anAcknowledgement frame and a Clear-to-Send frame from the first device inresponse to the Null frame.
 3. The method of claim 1, whereintransmitting the message requesting that the second device enabletransmission protection comprises broadcasting a management frame. 4.The method of claim 3, wherein the management frame is one of: (i) aBeacon frame indicating that protection status is active; and (ii) aProbe-Response frame indicating that protection status is active.
 5. Amethod comprising: transmitting, from an access point, a first messagerequesting that a first device enable transmission protection and asecond message requesting that the first device disable transmissionprotection, wherein the first message and the second message arecontinuously transmitted in an alternating pattern, and wherein a timeperiod separates the transmission of the first message and thetransmission of the second message; and in response to receiving amessage from a second device at the access point, adjusting the timeperiod separating the transmission of the first message and the secondmessage, wherein the second device is configured to communicate inaccordance with a first modulation scheme, and the first device andaccess point are configured to communicate in accordance with the firstmodulation scheme and a second modulation scheme.
 6. The method of claim5, wherein, if the most recent message sent from the access point to thefirst device is the second message, adjusting the time period comprisesreducing the amount of time until transmission of the first message. 7.The method of claim 5, wherein, if the most recent message sent from theaccess point to the first device is the first message, adjusting thetime period comprises increasing the amount of time until transmissionof the second message.
 8. The method of claim 5, wherein the firstmessage is a Beacon frame or a Probe-Response frame.
 9. The method ofclaim 5, wherein the message received from the second device is a legacymodulation frame.
 10. The method of claim 5, wherein the firstmodulation scheme is based at least in part on one of Barker modulationand Complementary Code Keying modulation; and wherein the secondmodulation scheme is based at least in part on Orthogonal FrequencyDivision Multiplexing modulation.
 11. An access point comprising: amemory comprising a computer-readable program; and a processor operablycoupled to the memory and configured to execute the computer-readableprogram to cause the access point to: determine a power save status of afirst device configured to communicate in accordance with a firstmodulation scheme, wherein determining the power save status comprises:transmitting a Null frame from the access point to the first device andawaiting a response to the Null frame from the first device, and inresponse to a determination that the first device is not in a power savestate, (i) enable transmission protection at the access point; and (ii)transmit, from the access point, a message requesting that a seconddevice enable transmission protection, wherein the second device and theaccess point are configured to communicate in accordance with the firstmodulation scheme and a second modulation scheme.
 12. The access pointof claim 11, wherein the access point requests transmission protectionat the third device by broadcasting a management frame via theshared-communications channel.
 13. The access point of claim 12, whereinthe management frame is one of: (i) a Beacon frame indicating thatprotection status is active; and (ii) a Probe-Response frame indicatingthat protection status is active.
 14. An access point comprising: amemory comprising a non-transitory computer-readable program; and aprocessor operable coupled to the memory and configured to execute thecomputer-readable program to cause the access point to transmit a firstmessage requesting that a first device enable transmission protectionand a second message requesting that the first device disabletransmission protection, wherein the first message and the secondmessage are continuously transmitted in an alternating pattern, andwherein a time period separates the transmission of the first messageand the transmission of the second message; and in response to receivinga message from a second device, adjust the time period separating thetransmission of the first message and the second message, wherein thesecond device is configured to communicate in accordance with a firstmodulation scheme, and the first device and access point are configuredto communicate in accordance with the first modulation scheme and asecond modulation scheme.
 15. The access point of claim 14, wherein, ifthe most recent message sent to the first device is the second message,the access point adjusts the time period by reducing the amount of timeuntil transmission of the first message.
 16. The access point of claim14, wherein, if the most recent message sent to the first device is thefirst message, the access point adjusts the time period by increasingthe amount of time until transmission of the second message.
 17. Anarticle of manufacture including a non-transitory computer-readablemedium having instructions stored thereon that, if executed by an accesspoint, cause the access point to perform operations comprising:determining a power save status of a first device configured tocommunicate in accordance with a first modulation scheme, whereindetermining the power save status comprises: transmitting a Null framefrom the access point to the first device and awaiting a response to theNull frame from the first device, and in response to a determinationthat the first device is not in a power save state, (i) enablingtransmission protection at the access point; and (ii) transmitting, fromthe access point, a message requesting that a second device enabletransmission protection, wherein the second device and access point areconfigured to communicate in accordance with the first modulation schemeand a second modulation scheme.
 18. The article of manufacture of claim17, wherein the access point requests transmission protection at thesecond device by broadcasting a management frame via theshared-communications channel.
 19. The article of manufacture of claim17, wherein transmitting the message requesting that the second deviceenable transmission protection comprises broadcasting a managementframe.
 20. The article of manufacture of claim 17, wherein themanagement frame is one of: (i) a Beacon frame indicating thatprotection status is active; and (ii) a Probe-Response frame indicatingthat protection status is active.